package com.ztom.dp;

/**
 * 第 N 个泰波那契数
 * <p>
 * https://leetcode-cn.com/problems/n-th-tribonacci-number/
 *
 * @author ZhangTao
 */
public class Code02Tribonacci {

    public int tribonacci2(int n) {
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 1;
        }
        return tribonacci2(n - 3) + tribonacci2(n - 2) + tribonacci2(n - 1);
    }

    public int tribonacci1(int n) {
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 1;
        }
        int[] dp = new int[n + 1];
        dp[0] = 0;
        dp[1] = 1;
        dp[2] = 1;
        for (int i = 3; i <= n; i++) {
            dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3];
        }
        return dp[n];
    }

    public int tribonacci(int n) {
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 1;
        }
        int first = 0;
        int second = 1;
        int third = 1;
        int res = 0;
        for (int i = 3; i <= n; i++) {
            res = first + second + third;
            first = second;
            second = third;
            third = res;
        }
        return res;
    }
}
